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Mr Webb, director of policy at the financial mutual Royal London, said: “If the Government is planning to force tens of millions of people to work to 68, 69 or even 70, then it should be transparent about its plans.

“This would be a huge shift and should be properly debated, not buried in a technical document seen only by specialists.”

Experts estimate the plans could save the Treasury about £240billion by slicing around £8,000 off a state pension entitlement.

As part of the pension age review under way by former CBI chief John Cridland, the Department for Work and Pensions approached the Government Actuary.

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Mr Webb says people need to properly prepare for a working life as long as 52 years

Analysts there were asked to set out a schedule for state pension age increases assuming the Government stuck to its previously-announced policy that people should spend two-thirds of their adult life in work and one third in retirement.

But in an unannounced and unexpected move, the Department also asked for figures on the basis that people would only spend 32 per cent of their life in retirement, rather than 33.3 per cent.

The 32 per cent figure is based on the experiences of those who reached state pension age in the last 20 years, and therefore fails to take account of more recent improvements in longevity.

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The new calculations could affect up to 30 million employees currently younger than 55

According to analysis by experts Willis Towers Watson, the new calculations could affect up to 30 million employees currently younger than 55.

If the Government is planning to force people to work to 70 then it should be transparent

Former pensions minister Steve Webb

Under the new scheme people born between 1962 and 1972 would see their pension age rise from 67 to 68.

Those born between 1973 and 1985 would have a pension age of 69 rather than 68 under the previous policy.

David Robbins, a senior consultant at the company, said: “If the Government needs to shore up the public finances, it would struggle to find a more politically painless way to take £8,000 off tens of millions of people.

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“News that the state pension age will rise to 67 by 2028 barely generated a squeak of protest.

“Maybe people never believed the state pension promise anyway, or maybe it is just too far away to worry about.

“Starting the increase to 68 as soon as the state pension age hits 67 means it will happen in time to catch the big bulge of future pensioners born during the mid-1960s baby boom. A higher pension age will also boost tax revenues if people respond by staying in work.”

Mr Webb said: “The Government is for the first time paving the way for people in the workforce up to the age of about 30 to have to work until they are 70.”

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The Chancellor said the Government was going to review spending to take account of growing longevity

He pointed out that last week Chancellor Philip Hammond had said the Government was going to review spending to take account of growing longevity.

This was “an additional indication that a more aggressive approach on the state pension age seems likely,” said Mr Webb.

The Department for Work and Pensions said: “This work forms part of our research ahead of the first state pension age review.

“It’s important that we have a clear understanding of how the current system is working for pensioners.”